As we learn more about how COVID-19 spreads, it’s clear that air flow is a big deal, and that’s why masks are effective at slowing infection. That’s a problem for offices and commercial buildings and some homes, because over the last couple of decades buildings have less ventilation. Instead, they recirculate air to save energy by not letting that cooled or heated air escape. But these buildings also end up recirculating viruses, especially if heating and cooling systems aren’t paired with really good filters that remove viral particles from the air. Molly speaks with Jeffrey Siegel, a professor of engineering at the University of Toronto who specializes in indoor air quality.